A major unresolved problem in the field of gene therapy is how to achieve the expression of a therapeutic gene in target cells where its effects are desired while avoiding its expression in non-target cells. The problem is especially acute when the transgene has the capacity to harm non-tumor cells and tissues, for example, where a suicide gene is used to destroy a tumor. Two basic approaches have been attempted: the transgene can be delivered in the form of a vector targeted specifically to certain types of cells (vector targeting; see, e.g., Peng and Russell, Cur. Opin. Biotech. 10: 454-457 (1999)) or the transgene can be cloned downstream of a cell type-specific promoter (transcriptional targeting; see Vile, et al, Mol. Med. Today 4: 84-92 (1998)). Targeting of vectors can also rely on physically administering them to a particular anatomical location, either by relying on the natural tropisms of the vectors or by engineering them to recognize a molecular target. A combination of these approaches offers the best hope for the systemic delivery of vectors to treat human disease. Vile, et al., supra.
A variety of cell type-specific promoters are known. Examples include promoters for tyrosinase (specific for melanoma cells and melanoctyes; see, Bentley, et al, Mol. Cell. Biol. 14; 7996-8006 (1994)), carcinoembryonic antigen (CEA, specific for colorectal cancer cells; see, e.g., Schrewe, et al., Mol. Cell. Biol. 10: 2738-2748 (1990)), alpha fetoprotein (specific for hepatocytes; see, e.g., Ghebranious, et al., Mol. Reprod. Dev. 42: 1-6 (1995)), erb-B2 (specific for breast cancer cells; see, e.g., Pandha, et al., J. Clin. Oncol. 17: 2180 (1999)) and myelin basic protein (specific for glioma cells; see, e.g., Shinoura, et al., Cancer Res. 59: 5521-5528 (1999)).
However, the use of cell type-specific promoters to induce the expression of cytotoxic agents in tumor cells is particularly problematic. The higher the potency of the suicide gene applied (e.g., toxicity), the greater the potential damage to non-tumor cells that receive the gene if the promoter controlling the suicide gene is not perfectly tumor-specific. Further, inadequate promoter specificity can have serious deleterious effects in non-target cells and tissues which are only revealed under certain conditions. For example, previous work has demonstrated that three tandem repeats of an enhancer element from the human tyrosinase gene (the tyrosinase distal element, TDE), when combined with a basal SV40 promoter, is sufficient to support highly selective expression of the cytokine GM-CSF in human melanoma cells (Diaz et al. J. Virol. 72: 789-95 (1998)). However, when the TDE-5V40 promoter is used to drive the expression of the envelope glycoprotein from Gibbon Ape Leukemia Virus (GALV), a highly cytotoxic fusogenic membrane glycoprotein (FMG), 3 of 9 non-melanoma cell lines showed significant amounts of cell killing (syncytium formation) after 72-96 hours, indicating that for this construct the TDE-5V40 promoter was not completely cell type-specific. Therefore, the high toxicity of proteins like GALV envelope glycoprotein, which makes them desirable as potent antitumor agents, will result in an unacceptable amount of bystander killing (killing of nearby normal cells) unless the expression of such agents can be made highly specific for their target cells.
While these problems can be overcome by developing promoters with higher specificity, high tissue specificity tends to be achieved at the expense of promoter strength, thereby undercutting the potency of the therapeutic gene.